This invention relates to self-aligning joints and to linkages employing self-aligning joints. Examples of linkages employing self-aligning joints include tie rod assemblies, drag link assemblies, torque rod assemblies, control rod assemblies, and many other applications in the automotive, agricultural, and industrial fields.
The most commonly used self-aligning joint is the ball and socket joint commonly called "ball joint." A ball joint generally consists of a mounting stud terminating in a ball and disposed within a socket and stem member such that the stud portion extends through an opening in the socket member which is smaller than the ball diameter. The ball has a slidable swiveling engagement with a conforming bearing surface in the socket. A preloading construction generally completes the joint assembly with the bearing surfaces in preloaded contact.
Three basic motions determine the amount of self-alignment which can be handled by a specific ball joint. The socket and stem member can be rotated freely around the axis of the ball stud, but the angular movement of the socket and stem in the plane of the ball stud axis is limited by interference between the opening in the socket and the ball stud. Excessive angulation can cam the ball through the opening in the socket body or break or bend the ball stud. To minimize the chance of "cam out" or stud breakage, the diameter of the ball stud can be reduced at the point of interference. This is common practice, but it reduces the load and fatigue limits of the ball stud.
The third motion of the ball joint is rotation about the axis of the socket and stem. This movement can create problems. For example, in some steering systems rotation about the socket stem axis due to the design geometry of the steering system is in the order of plus or minus 5.degree., occurring on a lock to lock turn of about 45.degree.. Additionally, the common axle forgings can oscillate as much as plus or minus 25.degree. metal to metal and this oscillation is superimposed on the design geometry oscillation. The resulting combined oscillation moves the socket body back and forth into contact with the ball stud and introduces a sense of lost motion and instability to the driver of the vehicle. Many proposals have been made to eliminate this third movement and create what is known as a "zero angle" ball joint. These proposals, however, are in general complicated and quite expensive.
Another major shortcoming of the ball joint is the difficulty in grease sealing. "Tent" type seals are the least expensive and are commonly used on automotive and agricultural applications. The seal fits snugly on the ball stud at the mounting surface and, upon installation of the joint, the seal is compressed against the exterior bowl of the socket body. The seal surface is a small annular band of contact which "gaps" with the large angular movements and, at best, grease retention is poor. It requires frequent greasing to purge contaminants and remain filled. The other type of seal is the bellows seal fitted and secured around both the ball stud and the bottom of the socket body. Angulation is handled well by the bellows except for large angulations which frequently pinch and rupture the bellows. The bellows is ruptured in many ways and its service life is seldom as long as the joint replacement. Power greasing systems fill so fast that the pressure relief cuts are insufficient and the seal blows off the socket body. At this point there is no effective sealing.
Ball joints are also very dependent on essentially perfect sphericity of the ball. The ball stud forging must be trimmed to eliminate heavy flash around the ball and the ball must then be burnished to obtain some degree of sphericity. At best, the ball is out of round. Under the heavy preloads required in ball stud assemblies, any variations in roundness imprints in plastic bearings resulting in initial lockup or very high breakaway steering torque.